Porous thermal isolation mesas for hybrid uncooled infrared detectors

ABSTRACT

Thermal isolation mesas 36 comprising a porous material 64 are used to thermally insulate sensing integrated circuitry 44 from pixels 34 of an uncooled IR detector hybrid system 30. The porous material 64 is preferably a silicon-dioxide xerogel. The mesas 36 may also comprise a protective film 66.

This is a division of application Ser. No. 08/412,662, filed Mar. 29,1995.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following U.S. patent applications are commonly assigned and arehereby incorporated herein by reference:

    ______________________________________                                                          Filing                                                      TI Case                                                                              U.S. Ser. No.                                                                            Date     Inventor Title                                     ______________________________________                                        TI-18788                                                                             08/182,865 1/13/94  Belcher et al.                                                                         Infrared                                                                      Detector and                                                                  Method                                    TI-17233                                                                             08/182,268 1/13/94  Belcher et al.                                                                         Infrared                                                                      Detector and                                                                  Method                                    TI-19178                                                                             08/255,157 6/7/94   Gnade et al.                                                                           Porous                                                                        Composites                                                                    as a Low                                                                      Dielectric                                                                    Constant                                                                      Material                                                                      for Electron-                                                                 ics Applica-                                                                  tions                                     TI-18996                                                                             08/263,572 6/23/94  Gnade et al.                                                                           Porous Di-                                                                    electric                                                                      Material                                                                      with Im-                                                                      proved Pore                                                                   Surface                                                                       Properties                                                                    for Electron-                                                                 ics Applica-                                                                  tions                                     TI-18941                                                                             08/247,195 5/20/94  Gnade et al.                                                                           A Low Di-                                                                     electric Con-                                                                 stant                                                                         Material for                                                                  Electronics                                                                   Applications                              ______________________________________                                    

The following U.S. patent application filed concurrently herewith thepatent application for the present invention is also incorporated hereinby reference:

    ______________________________________                                        TI Case                                                                              Inventor  U.S. Ser. No.                                                                            Title                                             ______________________________________                                        TI 19898                                                                             Beratan   08/412,817 High Thermal Resistance                                  et al.               Backfill Material for                                                         Hybrid UFPAs                                      ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to infrared or thermal imaging systems,and more specifically to thermal isolation in a thermal (infrared)detector array.

BACKGROUND OF THE INVENTION

Infrared or thermal imaging systems typically use thermal sensors todetect infrared radiation and produce an image capable of beingvisualized by the human eye. Some examples of such thermal imagingdevices include night vision equipment and law enforcement surveillanceequipment.

Several prior art references disclose infrared imaging arrays andmethods for producing such arrays. U.S. Pat. Nos. 4,080,532 issued toHopper; and 4,745,278 and 4,792,681 issued to Hanson utilizeferroelectric materials for infrared detection. Thermal imaging by meansof uncooled sensors is described in a paper entitled Low-cost UncooledFocal Plane Array Technology written by Hanson, Beratan, Owen andSweetser presented Aug. 17, 1993 at the IRIS Detector Specialty Review.

SUMMARY OF THE INVENTION

The present invention is a method for fabricating a hybrid thermaldetector structure, comprising the steps of providing integratedcircuitry, depositing a precursor film on the integrated circuitry,gelling the precursor film to form a porous film, patterning and etchingthe, porous film to form thermal isolation mesas on the integratedcircuitry, and forming interconnect metal on the thermal isolationmesas, where the interconnect metal is electrically connected to theintegrated circuitry. Then an infrared sensing array comprising at leastthree thermally sensitive pixels, electrical contacts abutting a firstside of the pixels, and an optical coating in contact with a second sideof the pixels is provided, and the interconnect metal is coupled to theelectrical contacts of the infrared sensing array. A protective film maybe deposited on the porous film after gelling the precursor film, toenhance the mechanical strength of the mesas.

The present invention also entails a hybrid thermal detector structure,comprising an infrared sensing array comprising at least three thermallysensitive pixels, electrical contacts abutting one side of the pixels,and an optical coating in contact with an opposite side of the pixels;and a sensing integrated circuit structure comprised of integratedcircuitry, a thermal isolation structure mounted on the integratedcircuitry, and interconnect metal electrically connecting the integratedcircuitry to a top region of the thermal isolation structure; where thethermal isolation structure is comprised of a porous material andwherein the electrical contacts of the infrared sensing array arecoupled to the interconnect metal of the sensing integrated circuitstructure. The thermal isolation structure may also comprise aprotective film.

Advantages of the invention include simplification of the lithographyprocess for forming the interconnect metal that provides electricalconnection between the pixels and the integrated circuitry, due to adecrease in the mesa height. Lower temperatures are required to processthe porous film, which prevents or minimizes damage to the underlyingintegrated circuitry. Thermal isolation between the pixels and theintegrated circuitry is improved because silica aerogels and xerogelsare better thermal insulators than organic materials used in the past.The porous mesas result in improved thermal isolation, and may permitthe use of thinner pixels in the future. The mesas may also comprise anoptional protective film which enhances the mechanical strength of themesas, creates a more planar surface for improved photolithography, andseals the surfaces of the mesas which minimizes outgassing in lowpressure applications.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which form an integral part of the specification andare to be read in conjunction therewith, and in which like numerals andsymbols are employed to designate similar components in various viewsunless otherwise indicated:

FIG. 1 illustrates the generic concept of a hybrid structure of theprior art consisting of a planar infrared sensing array electrically andphysically bonded to thermally isolating mesas on a sensing integratedcircuit;

FIG. 2 shows a precursor film deposited on sensing integrated circuit;

FIG. 3 shows the structure after the precursor film has been gelled toform a porous film on the integrated circuit;

FIG. 4 illustrates the structure after the porous film has beenpatterned and etched to form thermal isolation mesas;

FIG. 5 shows interconnect metal formed on the thermal isolation mesas,in contact with vias on the integrated circuit;

FIG. 6 shows a cross section of the hybrid structure having mesascomprised of a porous film, after the infrared sensing array has beenattached to the sensing integrated circuit structure;

FIG. 7 shows the structure after an optional protective film has beendeposited on the porous film;

FIG. 8 illustrates the structure after the protective film and porousfilm have been patterned and etched to form thermal isolation mesas; and

FIG. 9 shows a cross section of the hybrid structure having mesascomprised of a porous film and a protective film, after the infraredsensing array has been attached to the sensing integrated circuitstructure.

The drawings are neither to absolute nor relative scale. Thin filmthicknesses have been exaggerated for clarity in description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thermal imaging systems described in prior art references utilize anarray of ferroelectric or bolometric sensors to detect thermal sceneswhich may then be converted into a visual, for example, TV, image. Eachpixel (or resistor, when bolometric sensors are used) in these arrayscomprises a pyroelectric capacitor having a temperature-sensitivepolarization. Since the charge stored by a capacitor is a function ofits terminal voltage and capacitance, electronic circuitry may beattached to the two terminals of the pixel capacitor to measure theintensity of the infrared radiation impinging on a specific pixel.Obstructions in the imaging field are removed and electronic connectionsto these capacitors are simplified if one of these pixel terminals iscommon to all. From hundreds to hundreds of thousands of connections aremade between the other isolated terminals of the capacitors and theintegrated circuit used for electronic sensing. In addition, the pixelsare thermally isolated from each other while having one terminalelectrically connected to all the other common terminals.

The common connection, or common electrode, to one side of the pixelcapacitors may be part of an optical coating comprised of a plurality ofthin films having the desired physical properties, such as infraredtransparency, electrical conductivity, and thermal conductivity, forexample. The infrared energy is absorbed by the optical coating and istransferred to the pixels which may be made, for example, of bariumstrontium titanate (BST). The electrical polarization and capacitance ofa pyroelectric material such as BST changes in response to temperature.

Typically, an infrared absorber and common electrode assembly aredisposed on one side of the pyroelectric element and comprise an opticalcoating disposed over a common electrode. A sensor signal electrode maybe disposed on the opposite side of each pyroelectric element. Theinfrared absorber and common electrode assembly typically extend acrossthe surface of the focal plane array and electrically couple eachpyroelectric element through the common electrode. Each infrareddetector element or thermal sensor is defined, in part, by a portion ofthe infrared absorber and a common electrode assembly and a respectivesensor signal electrode, which constitute capacitive plates, and apyroelectric element, which constitutes a dielectric or insulatordisposed between the capacitive plates.

To maximize thermal response and enhance thermal image accuracy, eachpyroelectric element of a focal plane array is preferably isolatedthermally from adjoining pyroelectric elements so that the sensor signalaccurately represents incident infrared radiation associated with eachthermal sensor. When the uncooled IR hybrid structure is in use, thetemperature of the pixel is modulated by chopping incident IR radiation.This temperature change is sensed as a voltage by the readout IC. Foroptimum operating performance, it is also important to thermally isolatethe pyroelectric elements from the sensing integrated circuitry.

The making and use of the presently preferred embodiments are discussedbelow in detail. However, it should be appreciated that the presentinvention provides many applicable inventive concepts which can beembodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not delimit the scope of the invention.

The following is a description of preferred embodiments andmanufacturing methods of the present invention. Table 1 below providesan overview of the elements of the embodiments and the drawings. Thepresent invention and its advantages are best understood by referring toFIGS. 2-9 and Table 1 with like numbers being used for like andcorresponding parts in the drawings.

                                      TABLE 1                                     __________________________________________________________________________                                 Other Alternate                                  Drawing           Preferred or Specific                                                                    Examples or                                      Element                                                                            Generic Term Examples   Descriptions                                     __________________________________________________________________________    30   Hybrid Structure        Hybrid device; hybrid                                                         system                                           32   Optical coating                                                                            Multilayered                                                34   Pixels       Barium strontium                                                                         Thermally sensitive                                                titanate   dielectric; pixel                                                             dielectric;                                                                   pyroelectric                                     36   Thermal isolation mesas                                                                    Porous film 64                                                                           Porous film 64 and                                                            protective film 66                               38   Integrated circuit vias of                                                    integrated circuitry 44                                                  40   Interconnect metal                                                                         TiW        Au, other metals,                                                             conductive oxides.                               42   Infrared pixel electrical                                                                  NiCr       TiW; Au; a 4 layer                                    contact                 composite of:                                                                 In .5-6 μm                                                                 Au .01-.5 μm                                                               NiCr .01-.5 μm                                                             TiW .01-.5 μm                                 44   Integrated circuitry                                                                       Silicon technology                                                                       IR Sensing IC,                                                                Sensing integrated                                                            circuit, GaAs, readout                                                        IC                                               46   Infrared sensing array                                                                     Optical Coating 32,                                                                      Infrared imaging                                                   Pixels 34, Electrical                                                                    array                                                              contact 42                                                  48   Sensing integrated                                                                         Mesas 36, interconnect                                           circuit structure                                                                          metal 40, integrated                                                          circuitry 44                                                52   Infrared transparent                                                                       NiCr (50 Å)                                                                          25-100 Å                                          layer of 32.                                                             54   1/4 wavelength separator                                                                   Parylene (1.4 μm)                                                                     1/4 wavelength at                                     layer of 32             desired infrared                                                              signal; polyimide                                56   Electric conducting layer                                                                  NiCr (1000 Å)                                                                        500-2000 Å; common                                of 32                   electrode; cermet;                                                            other metals or alloys                                                        such as TiW                                      60   Bonding material                                                                           Indium alloy                                                62   Precursor film                                                                             Silica precursor film                                                                    Organometallic                                                                silicate solution; a                                                          liquid that will gel to                                                       form a SiO.sub.2 or SiC                                                       xerogel or aerogel                               64   Porous film  Silicon dioxide-based                                                                    Silicon dioxide-based                                              xerogel    aerogel; silicon                                                              carbide-based aerogel                                                         or xerogel; other                                                             aerogels or xerogels;                                                         preferably >80%                                                               porosity.                                        66   Protective film                                                                            Silicon dioxide                                                                          Silicon nitride; other                                                        dense thin films;                                                             other silicon                                                                 composites.                                      __________________________________________________________________________

While many different geometries and configurations have been describedin the references, FIG. 1 shows a prior art drawing of a generic hybridstructure 30 comprised of an infrared sensing array 46 electrically andphysically bonded to a sensing integrated circuit structure 48. Theoptical coating 32 is comprised of a partially infrared transparentlayer 52, a 1/4 wavelength filter material 54 and an electricalconducting layer 56. The electrical and physical mounting of infraredsensing array 46 to sensing integrated circuit structure 48 is achievedby the use of a bonding material 60 to adhere the infrared electricalpixel contacts 42 with the interconnect metal 40 through the vias 38.

There are several problems with the prior art structure depicted inFIG. 1. First, the interconnect metal 40 which provides the electricalconnection between the pixels 34 and the integrated circuitry 44 isdifficult to pattern and etch because of the height of the mesas 36which are typically around 12 μm tall. Second, the temperatures requiredto process the organic materials used in the past for the thermalisolation mesas 36 may damage the underlying integrated circuitry 44.The properties of resistors on the underlying IC are permanently anddeleteriously changed at the temperature range of 350°-400° C. that iscurrently used for polyimide. Third, although the pixels 34 have beenthermally isolated from integrated circuitry 44 by thermal isolationmesas 36 mounted to integrated circuitry 44, thermal crosstalk may stillresult via thermal conduction through the mesas 36 and interconnectmetal 40. Mesas 36 of prior art were comprised of an organic materialsuch as polyimide or PMMA (polymethylmethacrylate) which does notprovide sufficient thermal insulation between the pixels 34 and theintegrated circuitry 44, and restricts ultimate detector geometry.

A first embodiment of the present invention is shown in FIGS. 2-6. FIG.2 shows the sensing integrated circuitry 44 upon which a precursor film62 has been deposited. The precursor film 62 is preferably anorganometallic silicate solution which may be gelled into asilicon-dioxide based xerogel. The precursor film 62 may also compriseliquids that will gel to form silicon dioxide-based aerogel, siliconcarbide aerogel or xerogel, or other aerogels or xerogels. The precursorfilm 62 is liquid and is very planar when applied.

The precursor film 62 is gelled to form a porous film 64 as shown inFIG. 3. The precursor film 62 may be gelled by supercritical ornon-supercritical drying. Preferably, the resulting porous film 64 isgreater than 80% porous. However, the porous film 64 may also be lessthan or equal to 80% porous, so that the mechanical strength of theresulting porous film 64 is sufficient enough to structurally supportthe hybrid structure 30.

The porous film 64 is patterned and etched to form thermal isolationmesas 36 in porous film 64 as shown in FIG. 4. The mesas 36 may be ofshorter height (for example 3-6 μm, or one-quarter to half the height ofan organic mesa found in prior art) than that of prior art due to theimproved thermal insulative properties of the porous film 64.Interconnect metal 40 is then formed, making electrical connections tothe integrated circuitry 44 as discussed in prior art. The lithographyprocess for the interconnect metal 40 is made easier by the fact thetopography of the mesas 36 has been scaled-down, or reduced. Theinfrared sensing array 46 is then bonded to the sensing integratedcircuit structure 48 with the use of a bonding material 60 appliedbetween the infrared pixel electrical contact 42 and interconnect metal40, as illustrated in FIG. 6 to form the completed hybrid structure 30.(The infrared pixel electrical contact 42 is actually bonded to acontact pad connected to the interconnect metal, as described in thereferences).

A second embodiment of the present invention is demonstrated in FIGS.7-9. After the precursor film 62 has been gelled to form the porous film64, a protective film 66 is deposited over the porous film 64 as shownin FIG. 7. The protective film 66 is preferably silicon dioxide that issputtered on at room temperature. However, the protective film 66 mayalso comprise other silicon composites, or conventional oxides ornitrides. Next, the protective film 66 and the porous film 64 arepatterned and etched to form thermal isolation mesas 36. Each mesacomprises a bottom portion of porous film and a top portion ofprotective film, as shown in FIG. 8. Interconnect metal 40 is depositedand the infrared sensing array 46 is bonded to the sensing integratedcircuit structure 48 as described for the first embodiment to create thestructure shown in FIG. 9. The protective film enhances the mechanicalstrength of the porous film, resulting in strengthened mesas 36 forstructural support of the hybrid detector system 30.

There are many alternates to the hybrid structure illustrated. Forexample, although the optical coating 32 is represented as a planarsurface, this coating may contain elevations or corrugations for betterthermal isolation as has been shown in the references. The porous layermay comprise other suitable aerogels or xerogels. The protective film 66may also be applied after the mesas have been formed, so that theprotective film 66 covers all surfaces of the mesas.

The novel invention of thermal isolation mesas manufactured from aporous film has many advantages over prior art thermal isolation mesas.First, the lithography process, or patterning, of the interconnect metal40 which provides the electrical connection between the pixels 34 andthe integrated circuitry 44 is simplified because the height of themesas 36 is reduced to 3-6 μm. Second, the temperatures required toprocess the porous film 64 typically are below 100° C., which preventsor minimizes damage to the underlying integrated circuitry 44. Third,thermal isolation between the pixels 34 and the integrated circuitry 44is improved because silica xerogels and aerogels are better thermalinsulators than organic materials. The improved thermal isolationresults in better acuity of the image produced by the hybrid structure,and may permit the use of thinner pixels. In the future, thinner pixelsmay be utilized for hybrid infrared detectors. The use of the porousfilm of this invention for the formation of the thermal isolation mesasprovides increased thermal isolation and allows use of thinner pixels(although the interconnect metal material may need to be changed to ametal having a higher thermal resistance). The optional protective film66 enhances the mechanical strength of the mesas, creates a more planarsurface for improved photolithography, and seals the surfaces of themesas which minimizes outgassing in low pressure applications.

While the invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of theinventions, will be apparent of persons skilled in the an upon referenceto the description. It is therefore intended that the appended claimsencompass any such modifications or embodiments.

What is claimed is:
 1. A hybrid thermal detector structure,comprising:an infrared sensing array comprising at least three thermallysensitive pixels, electrical contacts abutting one side of said pixels,and an optical coating in contact with an opposite side of said pixels;and a sensing integrated circuit structure comprised of integratedcircuitry, a thermal isolation structure mounted on said integratedcircuitry, and interconnect metal electrically connecting saidintegrated circuitry to a top region of said thermal isolationstructure, said thermal isolation structure comprised of a porousmaterial; wherein said electrical contacts of said infrared sensingarray are coupled to said interconnect metal of said sensing integratedcircuit structure.
 2. The structure of claim 1 wherein said porousmaterial is a xerogel.
 3. The structure of claim 2 wherein said xerogelcomprises silicon dioxide.
 4. The structure of claim 1 wherein saidporous material is an aerogel.
 5. The structure of claim 4 wherein saidaerogel comprises silicon dioxide.
 6. The structure of claim 1 whereinthe porosity of said porous material is greater than 80%.
 7. Thestructure of claim 1 wherein said thermal isolation structure alsocomprises a protective film.
 8. The structure of claim 7 wherein saidprotective film comprises silicon dioxide.